1. Field of the Invention
This invention relates to steel sheets for DI (drawing and ironing worked) cans, and more particularly to tinned steel sheets for DI cans having improved DI workability, resistance to die abrasion and corrosion resistance.
Throughout the specification, the term "steel sheet" means a blackplate before plating or removing a plated layer after the plating, and the term "plated steel sheet" means a sheet obtained by plating the blackplate, and the term "DI can" means a two-piece can obtained by subjecting the steel sheet or plated steel sheet to drawing and ironing, and the term "DI work" means a working for forming the two-piece can by drawing and ironing, and the term "DI workability" means an easiness of DI work.
2. Related Art Statement
DI cans are used for filling carbonated beverage or other beverage. In turn, the plated steel sheet used for DI cans is produced by annealing a steel sheet after cold rolling, subjecting the annealed sheet to temper rolling through dull roll and then subjecting it to plating.
The tinning not only gives the corrosion resistance to the steel sheet but also serves as a lubricant in the DI work. Lately, the reduction of tinning amount is promoted for reducing the cost in the manufacture of DI cans.
In the plated steel sheet for DI can, it is required to have excellent DI workability and corrosion resistance and to reduce die abrasion, which are degraded as the tinning amount becomes small. That is, the reduction of tinning amount lowers the lubricating function to degrade the DI workability, and consequently the die life becomes short and the corrosion resistance is naturally degraded.
In order to solve these problems, there are proposed some methods. For instance, a method of producing plated steel sheets for DI cans by subjecting a steel sheet to temper rolling through dull rolls worked by discharge working or shot blast process and then tinning it is proposed in Japanese Patent laid open No. 54-150,331. This article discloses that when the surface roughness of the dull roll is less than 1.50 .mu.m, the shortage of tinning amount is caused during DI work to degrade the lubricating function, while when it exceeds 3.50 .mu.m, the working energy reduces but the appearance at the bottom of the DI can and the rust prevention are degraded. Further, the article discloses that when the cut-off level of the surface roughness is less than 1.20 .mu.m, the adhesion force of tin becomes small to lower the lubricating function, while when it exceeds 1.30 .mu.m, the adhesion force is certainly enhanced but the enhancing effect is undesirably small from a viewpoint of economical reason. And also, it discloses that when PPI (peak per inch) of the surface roughness is less than 200, the sectional area per peak becomes large and hence the hardening due to the ironing becomes conspicuous and the working energy unfavorably increases.
From the above facts, it is obvious that when the dull roll satisfies a surface roughness of 1.50.about.3.50 .mu.m, a cut-off level of 1.20.about.1.30 .mu.m and PPI of not less than 200, the resulting plated steel sheet for DI can has improved DI workability and die life.
Further, Japanese Patent laid open No. 55-158,838 discloses that the die life and stripping-out property are improved by limiting a surface roughness of a plated steel sheet for DI can to not more than of 20.mu. in (RMS) and applying an oil with a friction coefficient of not more than 0.12 to the sheet because as the surface roughness of the plated steel sheet becomes small, the tin covered ratio becomes large and the die abrasion reduces, while as the surface roughness becomes large, the stripping-out property is improved.
On the other hand, Japanese Patent laid open No. 55-50,485 discloses that as the surface roughness becomes large, the DI workability is improved but iron is exposed on the mountain portions of the plated steel sheet during DI work to degrade the corrosion resistance. In this article, therefore, the surface roughness (Ra) of blackplate (steel sheet) is restricted to Ra.ltoreq.0.4 .mu.m and a ratio of PPI at a preset level of 2 .mu.m to PPI at a preset level of 0.25 .mu.m is limited to not more than 0.05, whereby the above problem is solved.
As apparent from the above, these conventional methods are not a technique simultaneously satisfying all properties such as DI workability, resistance to die abrasion, corrosion resistance and so on. That is, if it is intended to simultaneously establish all properties by anyone of the conventional methods, the conflicting phenomenon is caused as follows.
As the surface roughness becomes small, since the mountain portion is small, the exposure of iron (blackplate) at the mountain portion in the DI work is prevented to improve the corrosion resistance and also the tin covered ratio increases to reduce the die abrasion, but the abraded tin powder produced in the working is caught in a contact face between the die (and/or punch) and the plated steel sheet to cause the galling because the abraded powder can not get away to the valley portions to degrade the trapping property. For this reason, the scuffing and baking are caused in the plated steel sheet to.degrade the quality and workability.
On the contrary, as the surface roughness becomes large, the working energy reduces to improve the DI workability and stripping-out property, but the tin covered ratio reduces to increase the die abrasion and the corrosion resistance is degraded due to the exposure of iron. When the surface roughness is too large, the degradation of appearance at the can bottom is caused in addition to the above problem.